1. Field of the Invention
This invention relates to illumination devices comprising light emitting diodes (LEDs) and, more particularly, to LED illumination devices that use phase-cut dimmers.
2. Description of the Relevant Art
The following descriptions and examples are provided as background only and are intended to reveal information that is believed to be of possible relevance to the present invention. No admission is necessarily intended, or should be construed, that any of the following information constitutes prior art impacting the patentable character of the subject matter claimed herein.
Lamps and displays using LEDs for illumination are becoming increasingly popular in many different markets. LEDs provide a number of advantages over traditional light sources, such as incandescent and fluorescent light bulbs, including low power consumption, long lifetime, no hazardous materials, and additional specific advantages for different applications. Mainstream usage of LED illumination devices has steadily increased over the years with advancements in LED technology and the resulting decreasing costs.
Many lighting applications use light dimmers to adjust the power delivered to the light sources, and therefore, control the intensity of light generated by the light source. Commercially available dimmers come in many different varieties with many different characteristics. Some dimmers comprise micro controllers, which typically are called electronic dimmers, while others comprise only passive components.
The vast majority of dimmers used in residential or commercial applications are phase-control devices (otherwise referred to as phase-cut dimmers), which were initially designed as a simple, efficient, and inexpensive method to dim incandescent light sources. Phase-cut dimmers, both leading-edge and trailing-edge, generally operate by limiting the power delivered to the load by conducting only a certain percentage of the AC waveform each half-cycle. In leading-edge phase-cut dimmers, the forward phase, or leading edge, of the AC waveform is removed from each half-cycle to limit the power delivered to the load. Conversely, trailing-edge phase-cut dimmers limit the power delivered to the load by removing the reverse phase, or the trailing edge, of each half-cycle. In both cases, slight dimming is achieved by removing a relatively small portion of the AC waveform, whereas a larger portion is cut for deeper dimming Manually varying the dimmer position varies the conduction angle and the conduction period, and hence, the power delivered to the load, resulting in a change in light output. Most phase-cut dimmers are wall-mounted devices powered by an AC mains line voltage of 120V RMS at 60 Hz or 220V RMS at 50 Hz.
FIG. 1 illustrates an example of a typical dimmer-controlled illumination device. Dimmer 10 is coupled to the AC mains line and produces a corresponding conduction angle at its output. An example of a rectified leading-edge conduction angle 12 is shown applied to a conventional power supply 14. If illumination device 16 is used to illuminate an LED load made up of one or more LED chains 18, then the power supply 14 typically includes an AC/DC converter that converts the phase-cut AC waveform at a manually adjustable conduction angle to a DC voltage (VDC). From the DC voltage, current of varying magnitude can be applied to the LED load 18 depending upon the brightness needed as well as the color spectrum desired if more than one red, green, blue, or white LED chain is used. Driver 20 can be used to drive the different LED chains to produce the desired brightness in lumens, and the different desired color spectrum.
FIG. 2 illustrates the conduction angle 12 of a leading edge phase-cut dimmer. It is well known that the conduction angle can be a trailing edge as well, and that conduction angle 12 is simply an example of one type of conduction angle formed by a phase-cut dimmer. The cross-hatch portion of the AC main waveform indicates the remaining phase-cut AC mains signal.
When used with an LED load, commercially available phase-cut dimmers provide inconsistent performance when dimming LEDs. One reason is in the design of an LED load versus an incandescent load. For example, an incandescent illumination device presents a simple resistive load with a linear response. Phase-cut dimmers work particularly well with this type of load, since the resistence of the filament decreases as its conduction angle decreases, resulting in naturally smooth dimming.
On the other hand, LED loads do not present a simple resistive load to the dimmer Instead, most LED loads can be characterized by a diode-capacitor power supply feeding a constant current source. The diodes rectify the applied AC voltage allowing it to charge the storage capacitor, while the LED loads draw a constant current from the power supply that is related to the desired dimming level and brightness. In the diode-capacitor power supply model of the LED load, current flows from the applied voltage to the load only when the magnitude of the applied voltage exceeds the stored voltage on the power supply capacitor, often coupled to the output of the power supply. The stored voltage on the power supply capacitor, in turn, depends on the current drawn by the LEDs themselves, which is a function of the LED brightness. Therefore, the current flowing from the power supply to the LED depends both on the instantaneous value of the AC voltage waveform and the brightness of the LED, which is dependent upon the dimmer conduction angle.
In conventional dimmer design, the current flowing to the LED load is related or relative to the conduction angle output from the dimmer. For example, in a single stage switched mode power supply, the energy storage element, either inductor or capacitor, must supply power to the LED while the triac dimmer, for example, is not conducting. As the conduction angle changes, the energy stored in the energy storage element (e.g., the diode-coupled capacitor or current-storage inductor at the output of the power supply), must therefore provide power for changing amounts of time. For example, as the conduction angle decreases, the energy storage element must provide power for increasing amounts of time. To keep the ripple current through the LED load relatively constant, the LED drive current through the LED load must decrease with decreasing conduction angle. The reverse is true if the conduction angle increases.
FIG. 5 illustrates the relationship between the dimmer conduction angle and the brightness of, for example, an incandescent load. Many dimmers have varying ranges of conduction angle that they can produce. Some produce conduction angles between 60° and 120°, while others can produce a wider range of conduction angles. As the dimmer is manually adjusted, either by rotating a knob or moving up and down a slider on a wall plate, the load responds accordingly; typically in linear fashion as shown. In the example of FIG. 5, the angle range from some dimmers may extend from 90° indicating maximum brightness to 45° indicating minimum brightness, while the angle range of other dimmers may extend from 165° downward to 15°; additionally, the angle range of some dimmers can change between the first time such dimmers are turned on and subsequent operation of those dimmers. For instance, some dimmers may first turn on with a minimum angle of 45°, but once on, will produce angles down to 30°.
As noted in conventional dimmer design, power supplied to the load, whether LED or not, is dependent on the conduction angle. If more brightness is needed, the conduction angle must be increased thereby increasing the power drawn from the AC main line and thus the load current supplied to the load. A power supply that produces the drive current to the LED load is therefore dependent on, and coupled to, the conduction angle output from the dimmer. It would be desirable to decouple the power supply from the conduction angle in certain instances where an LED load is used. For example, when different dimmers are used, it may be desirable to detect the differing conduction angle ranges of the newly attached dimmer, and adjust the mapping of the conduction angle to the brightness required by the LED load. In this way, the LEDs can adapt to whatever attached dimmer is used, so that the full mechanical range of a sliding or rotating dimmer can be employed to adjust to any desired LED brightness. Additionally, the LEDs and, more specifically, the LED drive currents applied thereto, can dynamically change the relationship between the input conduction angle and the brightness when attached to dimmers that have a different angle range when first turned on. As such, the LEDs will not “pop on” when such a dimmer is first increased from a minimum conduction angle setting.
Moreover, conventional LED illumination devices deliver power to the LED load proportional to the RMS voltage of the AC main, where the AC main can vary both in angle and amplitude. For example, those AC main voltages can vary by +/−20% or more from a nominal value causing the brightness of the LEDs to vary accordingly. Additionally, the minimum brightness is determined by the RMS voltage at the minimum angle from the dimmer. The minimum RMS voltage can be substantial, which then results in the minimum light output from the LEDs being quite bright, and barely less than a few percentage of the maximum brightness.
Most residential or commercial LED lighting applications come equipped with dimmers, and preferably triac dimmers. However, as noted above, coupling the unique characteristics of drive currents to LEDs and the attempted control of same using a dimmer coupled to the AC main line is problematic. While it is beneficial to retain the dimmer since most residential and commercial applications include a dimmer, it is also beneficial to remove LED output control from being controlled by the dimmer Thus, decoupling the dimmer conduction angle output from LED output is beneficial not only to enhance the range of LED output from that available using a dimmer but also to accommodate dimmers having differing conduction angles yet maintaining a more precise LED output control that that available using conventional dimmer designs. Most of all, it is of benefit to decouple the conduction angle from the power supply, which conventional dimmer-controlled illumination devices cannot achieve. However, if decoupling were to occur beyond what is currently available in conventional dimmer designs, the power supply would be able to control the minimum light output to be independent of the minimum conduction angle and to be arbitrarily small, for example, 0.1% of the maximum brightness of the LED output. This is much lower than what can be achieved using conventional dimmer-controlled illumination devices. Likewise, conventional dimmers that produce a relatively small maximum angle, for example, 90°, have correspondingly smaller maximum output brightness than those dimmers having a maximum angle greater than 90°. Decoupling the power supplied to the LED load from the conduction angle would enable the maximum brightness to be independent of the maximum conduction angle obtainable by the dimmer. This benefit not being one that a conventional dimmer-controller illumination device can achieve.
Although the RMS line voltage can vary with angle and amplitude, certain transients and drift can also be present from the output of a conventional dimmer-controlled illumination device. As shown in FIG. 3, the output of a leading edge phase-cut dimmer 10 (FIG. 1) can have certain transients 22 that occur when the line voltage is rectified initially to a relatively large voltage value with oscillations occurring on the leading edge of the conduction angle. In addition, at the conclusion of each conduction angle, leakage current through a triac for instance can cause the AC main line voltage into the lamp to drift, which can adversely affect the next conduction angle measurement. As shown, between conduction angles when a triac is supposedly off and the power supply is also off, small leakage currents may still flow through the triac. Leakage current causes upward DC drift 24 between conduction angles and, importantly, at the critical time in which the conduction angle is being measured by the power supply. If the triac resets prior to the AC mains rectified voltage dropping to near zero volts, the power supply might measure an incorrect conduction angle or may prevent the power supply from working properly. The combination of AC transients 22 and DC drift 24 can deleteriously affect measurements taken at power supply 14 coupled to receive the rectified AC main; thus, further affecting the brightness control on the LED load 18. As shown in FIG. 4, changes in conduction angle 26 can cause skew so that the corresponding brightness is not robust throughout the entire conduction angle range. In addition, transients and drift can affect the robustness of the brightness being controlled by the power supply.
In order to deliver smooth brightness control over a much wider range and to adapt to any conduction angle range of any attached dimmer, it would be desirable to introduce an improved power supply architecture. The improved power supply must be one that can decouple power delivered to the LED load from the conduction angle so that the power delivered derives from a source other than the dimmer and, thus, is independent from the conduction angle. The improved power supply can then adapt a power output to the LED load for any dimmer or conduction angle range of a dimmer applied to an AC mains line, and can operate at brightness levels much lower than conventional power supplies so as to dim a lamp to less than 0.1% of the maximum brightness of that lamp, for example. It is further desirable for the improved power supply to remove the AC transients and DC drift so as to achieve a more precise reading of the conduction angle, and also to know more precisely when to activate the power supply, and modify the DC power supply current at each conduction angle duration for more precise control of the drive current across a broader range of LED brightness.